The luminance in lux and the brightness or luminance in cd/m2 (luminance) can be directly measured with photometric devices.
It should be noted that errors in measuring devices of more than 10% are not uncommon. Therefore, the use of regularly calibrated measuring apparatus is a basic requirement.
Measurements are also very sensitive to environmental influences.

Some light theory:

With a big "Thank You" to Johan G. Smits, from www.Lichtconsult.nl, for the theoretical backup.

Luminance in lux

The illumination intensity of a specific surface area corresponds to the number of lumens that it receives per m2.
Unit: lux = lumens per m2 [lx]
Examples:
- Summer outside in the afternoon about 100,000 lx
- Office: average 400 lx
- Public lighting at busy points : approximately 30lx
- Public lighting in a quiet street : approximately 10lx
This value can be measured with a "Lux meter".

Luminous flux in lumens

The luminous flux is the total amount of light that a light source transmits per second in all the wavelengths of the visible light.
Unit: lumen [lm]
Examples:
- 100 W incandescent bulb gives approximately 1000 lm
- A candle gives approximately 10 lm

Luminous intensity in candela

The luminous flux is the total amount of light that a light source "transmits" per second in all the wavelengths of the visible light.
The light intensity of a light source, is the luminous flux per unit of an angle transmitted in a certain direction.
Unit: lumen = candela per star radial [cd]
1 star = radial surface of 1x1m in a sphere with a 1m radius,
wherein the vertices of the sphere are connected with the center of it.
Examples:
- Car headlight approximately 15,000 CD
- 100 W incandescent bulb about 80 CDs in each direction

Illumination

The light intensity of a lighting device can be determined by measuring the luminance (lux) in a plane perpendicular
to the direction of measurement and by the use of the following formulas:
E=I/d2 en I=E.d2
where,
E = illumination in the plane perpendicular to the direction of measurement in Lux,
I = the light intensity in the measuring direction, in candelas
d = the distance from the measuring point from the light source in the meter
Accordingly, the luminance (lux) of light decreases with the square of the distance.
E.g. a lamp of 400 Lux at 1 meter gives 400 Lux, 100 Lux at 2 m distance and 25 lux at 4 m.
So:
If we measure the amount of Lux with Lux meter at a distance of 1 meter, the measured Lux equals the light intensity
in Cd (Candela) or in good old English Cp, Candlepower.

Hands on !

Now we know the theory, we can move on to the practice.
The idea is simple : measure with a Lux meter the amount of Lux at a distance of one meter to our lantern.
But the sensor of a Lux meter is intended to measure ALL the incoming light, from all directions, while we only are interested in the direction perpendicular to our source.
We must therefore design a structure that absorbs or cuts off all other (reflected) light.
Assuming a sensor size of 45mm in diameter: Ideally A 50mm Dia tube with a length of almost one meter is required.
However, in practice, it appears that almost all materials have a degree of light reflection (luminance), and
therefore, because the sensor will not only see the light source it will also see the lateral reflections from the
inner surface of the tube, we measure more light than there actually is .....
Also a tube having a length of almost one meter is not easy to handle.
Without the use of exotic materials we have to create a structure which blocks the reflections while creating a
virtual non reflective pipe through which we measure.
It will look like this in the following picture:
This light meter set is one of three created by "The International Guild of Lamp Researchers" and is constructed
around the Isotech ILM350 Lux meter. Original design is from Neil Mcrae and Fil Graff
An other idea is to use PVC pipes:

To measure is to know.

In order to be able to compare measurements taken at different times and with different apparatus we need a
standard method of measurement. :
- At least 4 measurements in perpendicular directions, see fig.
- Pressure lamps should be operated with a pressure of 2bar in the tank. This is critical and the pressure needs to be as near to 2bar as possible.
- Minimum heating time of 30 minutes before measuring to get a stable light output,
- Special notes, like a built-in reflector, two mantles, variations in pressure Etc.
When measuring pressure lamps there is considerable fluctuation in the readings. It seems as if the light blinks or flickers. The best way to deal with this is
to measure the highest, lowest and record an average value.
This all sounds formal, but if we want comparable results then we must use the same parameters.
The table below gives the results of several measurements, in different directions of a number of known lamps.

Resultaten

`You cannot measure light with the mark 1 eyeball.`

Neil McRae, one of the three designers of this photometer

This statement by Neil McRae, who owns one of the three identical photometers, we can translate as :
"what we see is not the truth because the Mark I eyeball is not a calibrated instrument".....
Indeed, the results are not consistent with what one expects on the basis of what you think you see with the naked eye.

The entire table and all individual values are copyrighted !
Rated CP is the value specified by the manufacturer. We have not included a column recording the pressure for
pressure lamps,. All readings are taken at the manufacturer-specified optimum operating pressure.

For Tilley that is 30psi,
for Petromax (and clones) that is 2 bar.
Generally for other makes the pressure is also 2 bar but there may be exceptions in some early products that are
designed to work at under 2 bar and if there is any variation then that should be recorded in the notes